The present invention relates to a device for automatically replenishing toner to a dry-type developing apparatus for use in electrophotographic copying machines.
Developing apparatus of the subject type include in their upper portion a toner tank from which toner is dispensed at a constant rate usually by gravity or with a rotating brush, roller or the like provided at a toner outlet. The supply of toner is controlled generally from outside by mechanical means in accordance with the density of the observed copy images.
With such a dispenser, however, it is impossible to replenish the toner in proportion to the aggregate or charge on the electrostatic latent images which vary greatly with the denisty of the original. Thus when continuously developing electrostatic latent images of low aggregate charge, the quantity of toner within the developing apparatus increases to an excessive level, with the possible result that fog occurs in the non-image areas due to the deposition of toner thereon or the excess toner emits a fume to stain the interior of the copying machine. Conversely continuous development of electrostatic latent images of large aggregate charge will result in a deficiency of toner, producing copy images of reduced density or allowing carrier beads to adhere to the photosensitive element.
Accordingly a toner dispenser is required which will replenish toner in proportion to the density of the original, namely to the aggregate charge on the photosensitive element. Various devices of this type heretofore proposed include those which measure the density of the original images or copy images and control the supply of toner in accordance with the measurement, and those adapted to control the toner supply based on the amount of charge measured on the surface of the photosensitive element. The proposed devices nevertheless have the drawbacks of being complex, large-sized and expensive and failing the assure efficient and accurate control.
To overcome these problems, toner dispensers have been proposed in U.S. Pat. No. 3,453,045 and Japanese Laid-Open Patent Application No. SHO52-63739.
The dispenser described in the above U.S. patent comprises a guide plate for guiding the developer from a conveyor to the developing station and toner dispenser roller arranged for contact with the developer flowing over the guide plate and having the same triboelectric characteristics as the carrier. The roller, when driven in contact with a supply of toner in a toner tank, electrostatically attracts some of the toner on its surface and continuously brings the toner into contact with the developer on the guide plate to replenish the developer on the guide plate with the toner until the developer has been saturated therewith, namely until the amount of toner electrostatically attracted to the carrier has reached the saturation level. Thus the developer is fed to the developing station where it is always maintained at a constant toner concentration independently of the electrostatic latent images to be developed. The U.S. patent also proposed to control the optimum toner concentration by controlling the saturation level with an adjustable bias voltage applied to the dipenser roller and to thereby produce a variable contrast.
With the dispenser described in the laid-open Japanese application, the developer advanced on a developing sleeve or conveyor and passing through the developing station is wholly or partially directed by a guide member toward a toner recovering conductive roller to which is applied a bias voltage at a polarity opposite to that of the toner and which contacts the developer flowing on the guide member to electrostatically attract toner from the developer to the roller surface. The toner continuously attracted and recovered by the roller by virtue of its rotation is received in a toner tank. The developer is continuously or intermittently replenished with toner which is dispensed from the toner tank by another roller, such that when the developer has an optimum toner concentration, an amount of toner, equal to the amount F of toner recovered by the conductive roller within a specified short period of time To, is replenished from the toner tank by the dispenser roller during each timer period To. The amount F is made equal to Mt+F.sub.1 wherein Mt is the average amount of toner consumed by the development within the specified time To, and F.sub.1 is the amount of toner recovered by the conductive roller from the developer within the time To after the amount Mt has been consumed. In this way, a constant amount of toner is replenished (during the period of time To) independently of the electrostatic latent images to be developed. It is also explained that when Mt+F.sub.1 differs from F depending on the kind of original, the value Mt+F.sub.1 is made equal to F by controlling the bias voltage value on the conductive roller and/or adjusting the speed of rotation of the roller.
However, both the proposed devices leave much to be desired,
Whereas the toner dispenser roller of the former must be provided with the same triboelectric characteristics as the carrier, it is difficult for the roller to maintain such characteristics over a prolonged period of time since toner adheres on the roller surface and consequently impairs the triboelectic function of the roller. This results in a reduction in the amount of charges on the toner per se, giving rise to difficulties in maintaining the initial toner concentration. The bias voltage, if controlled, is unable to obviate this problem.
With the latter, the relationship of F=Mt=F.sub.1 could be maintainable insofar as fluctuating factors such as the kind of original and the frequency of the copying operation are held at average levels within the specificed time period To, but the definite relationship involved in the toner supply F is fundamentally difficult to maintain in an actual operation which handles images having changing factors. In fact, the concentration of the toner to be conveyed to the developing station inevitably varies over a wide range.
Consequently when the density of copy images has greatly changed in the course of a copying operation, there arises the necessity of adjusting the toner concentration by controlling the bias voltage and/or the speed of rotation of the recovery roller as in other conventional devices. Moreover, such adjustment is frequently necessary.
Further characterizing the foregoing mechanisms, are that the former requires a guide plate, whereas the latter requires a guide member and both a toner recovery roller and a toner dispenser roller. Either of the proposed constructions therefore necessitates provision of the guide plate, or the guide member and toner recovery roller as additional means and is accordingly more complicated than is desired.